This invention relates, in general, to solid state pressure sensors, and more particularly, to structures for sensing high pressures.
Solid state pressure sensing devices are well known and are used in many applications today based on the piezo-resistive characteristics of silicon. High pressure sensing devices, that is, devices capable of sensing pressures up to approximately 14,000 kiloPascal (kPa) (approximately 2000 pounds/square inch (psi)), typically comprise a silicon die that has an etched cavity to form a diaphragm. The silicon die is housed in a package that includes a metal barrier that separates the silicon die from a high pressure environment. The metal barrier is spaced a distance away from the silicon die to form a chamber. The chamber is filled with a silicone oil, which together with the metal barrier acts as a transfer media.
When the high pressure sensing device is exposed to a high pressure environment, the metal barrier is displaced toward the silicon die causing a displacement in the silicone oil and ultimately a displacement in the diaphragm. When the diaphragm is displaced, an electrical output signal is generated having a magnitude that is a function of the amount of stress in the piezo-resistive material.
Existing high pressure sensing devices have several disadvantages. First, they use a cavity etched silicon die, which is susceptible to stress related failures because of the fragile nature of the etched diaphragm. Also, expensive processing steps are necessary to form the cavity in the silicon die. In addition, existing high pressure sensing devices have a limited temperature operating range because of the thermal expansion characteristics of the silicone oil. At elevated temperatures, the silicone oil expands and applies a pressure against the metal barrier and the external pressure applied to the metal barrier, which causes the silicon die to sense an inaccurate external pressure. Additionally, the silicone oil is incompatible with many die attach materials, which results in reliability problems. Furthermore, existing high pressure sensing devices require manufacturers to control the tolerances of the metal barrier, the silicone oil, and the silicon die in order to fabricate reliable devices on a large scale production basis with minimal device-to-device variability. This greatly increases manufacturing costs.
Consequently, a need exists for a high pressure sensing device that is capable of accurately measuring pressures up to 14,000 kPa range, that does not utilize a cavity etched semiconductor die, that eliminates the need for a metal barrier/silicone oil transfer media, and that is cost effective.